The overall objective is to investigate the molecular mechanisms and genetic control of cellular repair of damaged DNA and to determine the effect of cellular repair mechanisms on genetic recombination, on DNA replication and on the induction of nuclear and mitochondrial gene mutations. These problems are being studied in a simple eucaryotic organism, the yeast Saccharomyces cerevisiae, where both mutants with repair deficiencies exist and a well-defined mutational system is available. Our approach is to utilize biochemical, molecular biological and genetic means to achieve these objectives. We are determining which genes are involved in dimer excision by measuring the susceptibility of UV-irradiated yeast DNA obtained from various rad mutants to nicking by UV-specific endonucleases. We will also identify and characterize the enzymes which participate directly in the repair of DNA damaged by various agents such as UV and ionizing radiation and various alkylating and other chemical mutagens and carcinogens. Studies on DNA polymerases will be continued to identify their genetic loci and to study their role in DNA repair, replication, recombination and mutation. The ability to repair X-ray and MMS-induced damage is being studied in mutants sensitive to these agents: the rad50 and rad57 mutants, the rad mutants involved in error-prone repair, and our newly isolated mms mutants. We are determining the size of the DNA synthesized following UV irradiation in various rad mutants to see whether post-replication repair plays an important role in yeast, which genes are involved, is it composed of both error-prone and error-free components and is any component of it inducible? We are continuing our studies on the genetic control of mutation for both nuclear and mitochondrial genes, determining the effect of the genes rad50 to rad57 on chemical mutagenesis. Our studies with other genes involved in repair, such as the mms genes, will indicate whether they play a role in mutation induction. Our investigations on the induction of both nuclear and mitochondrial mutations in rad and mms mutants will indicate to what extent there is joint control of mutation in these two types of DNAs.